Electrochemical Oxidation of Single-Walled Carbon Nanotubes in Aqueous Electrolytes Containing Various Types of Anions

The progression of oxidation upon applying positive potentials in five types of aqueous electrolyte solutions (KOH, K2SO4, KCl, KBr, KI) was investigated through Raman scattering experiments, XPS measurements, TEM observations, and other methods for three types of SWCNTs with average diameters of 1.0, 1.4, and 1.5 nm. It became evident that smaller-diameter samples undergo more significant oxidation when compared based on average diameter. In SWCNT samples with an average diameter of 1.0 nm, an increase in the Raman D band was observed starting around a 0.6 V application with an Ag/AgCl reference in KOH electrolyte. However, in other electrolytes, an increase in D band intensity generally occurs at around 1 V. The susceptibility to oxidation depending on the type of electrolyte was generally determined to be in the order of KOH ≈ K2SO4 > KCl > KBr > KI, based on the enhancement of the D band at the 1.4 V application point. XPS measurements revealed that oxidation progresses through the introduction of oxygen functional groups during electrochemical oxidation, and differences in oxidation due to the type of electrolyte are influenced by the oxidation of oxide ions by anion species. Even within the same sample, oxidation was confirmed to progress more in smaller-diameter SWCNTs based on changes in RBM peak intensity. Furthermore, experiments involving variations in Raman excitation light within the same SWCNT sample and comparisons of G band profiles revealed that metallic tubes are more prone to oxidation than semiconductor tubes. The changes in RBM profiles before and after oxidation can also be explained by differences in tube diameter and metal–semiconductor nature. No clear evidence was found regarding differences in oxidation susceptibility based on chiral angles, as discussed in the literature on the chemical oxidation of SWCNTs. Additionally, the chemical oxidation of SWCNTs by hydrogen peroxide treatment was investigated. Similar to electrochemical oxidation, it was confirmed that smaller-diameter samples are more susceptible to oxidation. Differences in oxidation susceptibility due to the metal–semiconductor nature similarly resulted in the finding that metallic tubes are more prone to oxidation, as observed in electrochemical oxidation.